1. Field
Aspects of the present invention generally relates to a radiation generating unit which is used for such as non-destructive X-ray photography in a medical device field and an industrial device field, and a radiography system which uses the radiation generating unit.
2. Description of the Related Art
Generally, by applying a high voltage between a cathode and an anode installed in a radiation tube, a radiation generating unit irradiates an anode with electrons discharged from the cathode and produces radiation rays such as X rays. Such a radiation generating unit adopts a structure in which a radiation tube and a driving unit are accommodated in a container filled with an insulating liquid to cool the radiation tube and to secure the dielectric strength against a high voltage.
When electrons discharged from a cathode are incident on an anode, efficiency to generate radiation rays is poor, and therefore almost all incident energy is converted into heat in the radiation generating unit. The heat generated in the anode is conducted to a radiation tube wall, transmits to the insulating liquid and is finally released to an external atmosphere through a storage container from the insulating liquid.
However, a flow of the insulating liquid in a wide range and effective transfer of heat of a high temperature portion to a low temperature portion are important to sufficiently cool the vicinity of an anode and release heat generated at the anode to an outside through the storage container.
Further, high voltages of about 70 to 150 kV are applied to both poles of a radiation tube. Hence, even when a container is filled with an insulating liquid, creeping discharge occurs at a surrounding portion of the radiation tube in some rare cases, and, when reaching a driving unit, this discharged electricity damages circuits.
Japanese Patent Application Laid-Open No. 2012-28093 discloses an X-ray generating device in which an X-ray tube is arranged in an insulating outer casing tube provided with multiple holes and with a gap in the surrounding such that insulating oil can freely flow. The insulating oil has a function of preventing discharge of electricity and cooling the X-ray tube.
As described above, although, since a radiation tube is covered by an insulating material in a conventional radiation generating unit, an effect of preventing creeping discharge to a circuit substrate can be expected, an effect of preventing micro creeping discharge of electricity generated near a surface of the radiation tube and in an electric field direction between a cathode and an anode is not provided. Therefore, development of tracking of a radiation tube causes damage on the radiation tube.